The United States will deploy an additional 1,000 troops in Poland, said President Donald Trump at a press conference with Polish Prime Minister Andrzej Duda.
President Trump said the force would be taken from the US contingent of 52,000 people in Germany, and would include drones and other military equipment.
However, he failed to engage in a permanent US base in the country.
It comes after Warsaw offers to spend $ 2 billion (£ 1.57 billion) to build one.
The base could be called Fort Trump, said President Duda during his visit to the White House on Wednesday.
President Trump said that the United States was "very interested" in this idea, but that they were reluctant to engage in a permanent facility – which would likely prompt a response from Russia.
"I'm not talking about permanence or non-permanence," he told reporters, adding that the base "would certainly be a statement".
The visit – Mr. Duda's second in less than a year – celebrated the 20th anniversary of Poland's accession to NATO and the 30th anniversary of the fall of communism in the country. .
Sending to reporters, Mr Duda thanked Mr Trump for his "extreme kindness towards Poland and his perfect understanding of Polish issues".
After leaving Washington, the Polish president has to travel to Texas, Nevada and California for talks on energy and technology.
For a year now, the Warsaw government has been pressuring the Americans to establish a permanent military base in Poland that can accommodate up to a division (several thousand) of US soldiers.
The idea was quickly dubbed "Fort Trump". But there were problems.
Who would pay? Warsaw has offered up to 2 billion dollars, but this would only cover the initial establishment of the base.
Where would the troops come from? Moving them to the United States would involve huge costs; moving from Germany or Italy could harm the cohesion of the alliance.
Above all, a permanent base could violate the 1997 agreement between NATO and Russia.
What happened is a fudge. Fewer troops than requested, and again by rotation. American commanders see a reinforcement of their preparation.
But these rotating troops will help develop Poland's military infrastructure in order to be able to receive a much larger number of soldiers if needed in the future.
What did the United States and Poland accept?
The two leaders signed a joint declaration affirming countries' defense cooperation.
The agreement includes provisions allowing Poland to provide "a base and infrastructure for 1,000 US troops", which would be temporarily rotated.
During his subsequent interrogation by the press, however, Mr. Trump said "they are talking about 2,000 soldiers".
When addressing the BBC, a US Department of Defense official was unable to confirm how many people would be sent to Poland under the new agreement.
It is also difficult to know who would bear the costs of the last US deployment.
About 5,000 soldiers are already doing rotations in Poland. This is part of a 2016 NATO agreement reached in response to Moscow's annexation of the Crimea of Ukraine in 2014.
In a separate agreement, Poland has ordered 32 F-35 fighter jets, which are in the United States by Lockheed Martin.
Shortly before the press conference, in an unusual presentation, two F-35s flew over the White House to show their movements.
Mr. Trump hailed them as "the world's largest fighter aircraft – the most advanced aircraft, probably, anywhere in the world beyond the jet fighter; the most advanced ".